Typically, air to be supplied to a passenger compartment is conditioned using HVAC (heating, ventilation, and air conditioning) systems equipped with a refrigeration system for cooling the air and a coolant/air heat exchanger of an engine cooling circuit for heating the air. Additionally, conventional HVAC systems typically include air/air heat pumps and/or coolant/air heat pumps.
In HVAC systems including a coolant/air heat exchanger, the system obtains heat from a cooling circuit of an internal combustion engine of the vehicle drive and conducts heat into the passenger compartment. The heat loss of efficient internal combustion engines is not sufficient at low ambient temperatures to heat a coolant to a temperature level required for comfortably heating the passenger compartment. The temperature level of the coolant, hence the heat flow into the air to be supplied to the passenger compartment, additionally depends on the operating conditions of the vehicle drive (e.g. running conditions of the vehicle such as the rotational speed and the load conditions of the engine).
Heat is delivered from the coolant to the air to be conditioned by means of a heating heat exchanger of the HVAC system. The transferred heat is adjusted either by controlling the air mass flow passing the heating heat exchanger or by controlling the coolant mass flow by-passing the heating heat exchanger.
Currently, in state-of-the art, two main types of HVAC systems are known.
The first type of HVAC system includes air flow controlled systems, wherein the air mass flow to be supplied to the passenger compartment is split into two partial air mass flows using a door. One of the partial air mass flows is directed through the heating heat exchanger and heated. The second partial air mass flow concurrently bypasses the heating heat exchanger. Both differently tempered partial air mass flows are then mixed in order to reach the required target temperature. However, mixing of the partial air mass flows is not optimum. At intermediate positions of the temperature door the air mass flows entering the passenger compartment through the various air outlets have different temperatures. The difference of the temperatures of the air flows at the various air outlets is known as thermal stratification. In certain situations, thermal stratification is desired such as providing a warm footwell and cooler headspace of a passenger compartment, else the comfort of the passengers would distinctly be reduced. Generally, the stratification depends on the position of the temperature door. In a “fully warm” position, all air flows passing through the outlets into the passenger compartment have the same temperatures so that there is no stratification.
As an example, such an HVAC system is disclosed in DE 10 2004 039 852 A1. The HVAC system includes a heating device for heating the air to be supplied to the passenger compartment. The heating device is supplied by a heating medium, which in turn is heated by a heating source. Using an air mixing door the air mass flow is directed through the heating device and/or bypasses the heating device. The air channel is equipped with a face outlet and a foot outlet.
The second type of HVAC system includes a coolant systems controlled on the coolant-flow side. In such systems, a coolant mass flow is controlled using a valve in such a way that the transferred heat is adjustable in proportion to the coolant mass flow for a given temperature. Depending on the kind of the heat exchanger, undesired thermal stratifications over the surface of the heating heat exchanger may result. In addition, the discontinuities of the coolant mass flow due to varying engine speeds produce other unintended effects.
In both different modes of operation of the HVAC systems (i.e. the air side- and the coolant side-controlled systems), the heat that is not transferred in the heating heat exchanger to the air to be conditioned (i.e. the unused heat) is transported as hot coolant to the vehicle radiator and dissipated to the environment. In vehicles that are traditionally engine driven, a relatively large amount of heat is dissipated to the ambient air in form of waste heat.
Further, in coolant side-controlled systems the varying coolant mass flow, as well as the varying coolant temperature, cause energetic losses, particularly when, due to timing, the coolant mass flow passing the heating heat exchanger is reduced.
Against the background of complete electrification of the drive or, respectively, the increasing use of vehicle drive systems with very little waste heat, such as electric drives, fuel cell drives, or hybrid drives, the heat available in the coolant for conditioning the air to be supplied to the passenger compartment will be markedly limited due to the higher efficiencies of the drive components.
At the same time, the amount of energy that is storable in the battery of the vehicle today is less than that storable in form of liquid fuel in the fuel tank. Accordingly, the power required for the air conditioning of the passenger compartment of an electrically or partial-electrically driven vehicle also significantly influences the range of the vehicle.
Therefore, energy losses caused by admixing cold air or dissipating heat to the environment in the radiator should be avoided.
Further, in electrically or partial-electrically driven vehicles, due to the little amount of available waste heat of the drive components, the temperature level of the coolant compared with traditional engine drives are clearly lower. An air side-controlled HVAC system, therefore, is preferably operated at the “fully warm” position of the temperature door so that no temperature stratification of the air flows at the outlets develops. However, the comfort of the passengers is distinctly reduced.
Coolant/air heat pumps known in prior art also use the coolant of the engine as heat source, whereby heat is extracted from the coolant. Therefore the engine is operated at low temperatures over prolonged periods, which negatively affects the exhaust gas emissions and fuel consumption. Due to the intermittent operation of the engine in hybrid vehicles, a sufficiently high coolant temperature will not be reached on longer rides. Therefore, at low ambient temperatures the start/stop operation of the engine is interrupted, wherein the engine is not switched off.
In electrically or partial-electrically driven vehicles, the electric drive has high efficiencies at low temperatures so that the drive is operated at a low temperature level, as compared with the engine. Also, the batteries, especially lithium ion batteries, have low temperatures of 40° C. maximum, which are not sufficient to directly air condition the passenger compartment, but are suitable to be a heat source for a coolant/air heat pump.
Against the background of the systems known in prior art, it is necessary to additionally use concepts of auxiliary heating in the HVAC system.
It is desirable to provide a method for the control of an efficient HVAC system that, independent of the operational conditions of the vehicle drive, (i.e. the running conditions of the vehicle), ensures a temperature stratification, as required for reaching and maintaining comfort, of the air mass flow to be supplied to the passenger compartment and provides a necessary heat as needed.